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The Cap-Rouge, Quebec, Earthquake
Although technically this month's geological "site" is out of state, the effects of the November 5 Cap-Rouge earthquake were felt by people across the State of Maine. We thank everyone who took the time to complete and return an "earthquake questionnaire" to us. Over the past four weeks, we have received and evaluated over 200 responses, and the cards continue to come in. If you felt the earthquake in Maine and have not yet completed a questionnaire, it is not too late to do so. We would like as many first-hand reports as possible. You may either complete our on-line form or request an earthquake questionnaire card by contacting the Maine Geological Survey. The Geological Survey of Canada has been collecting similar information in Canada for this earthquake.
As expected, the intensity of ground motion decreased away from the earthquake epicenter. The strongest effects, commonly at MM IV, were reported for northwestern Maine, through the Jackman-Greenville-Millinocket area and extending south to Farmington and east to Brewer. Intensities of MM III were commonly felt across Oxford, Androscoggin, Kennebec, and Waldo Counties and as far southeast as Hancock County. Remaining parts of the state in southern, eastern, and northern Maine generally experienced intensities of MM II. A large number of responses came from the Augusta-Belgrade area and from the Gorham-Westbrook-Topsham area. In Canada, the maximum intensities for this event were MM VII immediately around the epicenter, causing thousands of dollars of damage to a school.
On closer inspection of the Maine map you can see that, within the regions of generally similar intensities, there is a significant variability among individual sites. For example, there are some reports of MM IV intensity in the region where most places experienced MM II effects, and vice-versa. In other words, the intensity of motion experienced at a place depends on more than just the distance from the epicenter. This is because certain types of natural geologic materials amplify ground motion caused by seismic energy. In general, the intensity of ground motion in a given area is lowest on solid bedrock and greater on thick deposits of clay, sand, or artifical fill. The relationship between surficial geologic materials and intensity is a complex one due to the complicated structure and variety of earth materials. The data gathered on earthquake questionnaires over several events through the years will gradually help us to better understand this relationship.
Also notice on the intensity map that the isoseismal lines are not concentric circles centered around the epicenter at Cap-Rouge. The pattern is an irregular, roughly oval shape, in which higher intensities extend farther southeastward into central Maine, and die out sooner eastward into Aroostook County. Roughly oval-shaped patterns of isoseismal lines are typical for earthquakes in the northeast Appalachian region, and reflect the variable attenuation of seismic waves as they travel in different directions through the bedrock crust. (For example, see the isoseismal map for the 1988 Saguenay, Quebec, earthquake)
The Canadian National Seismograph Network, has seismic monitoring instruments deployed in the Quebec City - St. Lawrence region that recorded the earthquake. By combining data from seven seismic stations, they determined that the earthquake occurred at 9:34 P.M. (EST) on Nov. 5, 1997, with a magnitude of 5.2 (MN), at a depth of 22.5 km. If an earthquake of this magnitude had occurred close to the surface, the damage (or intensity) near the epicenter would have been greater.
Modern instrumental monitoring of earthquakes in Maine goes back only to 1975. Events before that time are known mainly from anecdotal information such as newspaper articles, letters, and diaries. For example, research on anecdotal reports from the April 4, 1904, earthquake near Eastport has allowed a map of intensity and isoseismal lines to be constructed. From that map, Leblanc and Burke (1985, Seismological Research Letters, v. 56, p. 107-124) have estimated a magnitude of 5.9 for the 1904 earthquake. This was probably the largest earthquake in Maine in the past 200 years, and is significantly larger than anythng recorded by modern seismic instruments. In order to make these types of magnitude estimates for historical earthquakes, the relationship between intensity and magnitude must be known. This, in turn, depends on collecting intensity information from people who experience modern earthquakes. Again, we thank all those who have responded.
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Originally published on the web as the December 1997 Site of the Month.
Last updated on April 27, 2012
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